A logarithmic detector provides an output signal that represents the logarithm of an input signal. Such a circuit is used in technical areas where it is important to process wide dynamic input ranges, for instance, in cellular radios, spectrum analyzers, cordless telephones and radar equipment. The logarithmic detector typically functions as a received signal strength indicator (RSSI). In cellular radio, this function is necessary for continuously monitoring the received signal strength by the radio's microcontroller. The RSSI function requires a dc output voltage which is proportional to the logarithm of the input signal level.
Basically, such a detector comprises a cascade of signal amplifiers. For the sake of clearness it is assumed below that the amplifiers have identical linear-amplification ranges and identical amplification "A". The first amplifier of the cascade receives an input signal "v" that is successively amplified by the subsequent amplifiers in the cascade. The repetitive amplification continues until a particular amplifier runs out of its linear range and into its limiting range. In its limiting range, each amplifier furnishes a clipped version of its amplified input signal. The n-th amplifier along the cascade runs into its limiting range when its output signal A.sup.n v exceeds the limit of the linear amplification range. Consequently, the input signal values at which the respective amplifiers leave their linear amplification ranges are proportional to A.sup.-n. Summing the output signals of all amplifiers approximates a linear function of the logarithm of the input signal.
In practice, the amplifiers are signal voltage amplifiers. The signals supplied by the cascade have to be rectified in order to generate a dc output quantity representative of the signal's strength. Generally, an inter-stage filter is inserted into the cascade for reducing noise. Such a filter, however, causes undesired losses in amplitude. The output voltages therefore usually are rescaled for loss-compensation before being processed further.
In prior art circuit NE605 manufactured by Signetics Co., a part of Philips, the output voltages of the amplifiers are routed to a rectifying element via a resistor ladder that comprises stacked pairs of resistors. The amplifiers have complementary outputs connected to nodes between particular pairs of resistors. The voltage division attained in this way takes care of the rescaling of the signal. The rescaling, however, also affects the various dc levels of the voltages at the ladder nodes. These rescaled dc levels are to be removed from the rectifier output signal in a subsequent circuit.
The supply voltage required for operating the prior art circuit therefore is determined, among other things, by the resistor ladder. Further, removing the dc levels from the rectified signal requires additional circuitry. As a result, the detector has to be redesigned for each particular combination of rescaling resistors. Also, since various resistor values are used in a ladder, the accuracy of the resistors should be well controllable in order to avoid accumulation of signal distortion in the ladder and beyond.